Natural gas is transported long distances in a gaseous state to consumers through a gas pipe line over land or sea, or is transported in a liquefied gas (LNG or LPG) state by carriers. Liquefied gas is obtained by cooling natural gas to a cryogenic state (about −163° C.) where the volume of the natural gas is reduced to about 1/600 of that at standard temperature and pressure, which makes it eminently suitable for long distance marine transportation.
An LNG carrier is designed to transport LNG at sea to consumers on land and includes liquefied gas storage tanks capable of sustaining the cryogenic temperature of the LNG. The storage tanks arranged in the LNG carrier can be classified into independent type storage tanks and membrane type storage tanks according to whether load of a cargo directly acts on a heat insulating material.
The independent type storage tank includes an SPB type tank and a Moss type tank, which are generally fabricated using a large quantity of non-ferrous metal as a main material, thereby causing a significant increase in manufacturing costs. Currently, the membrane type storage tanks are generally used as the liquefied gas storage tank. The membrane type storage tank is relatively inexpensive and is verified through application to the field of liquefied gas storage tanks without causing safety problems for a long period of time.
The membrane tanks are classified into a GTT No. 96 type and a Mark III type, which are disclosed in U.S. Pat. No. 5,269,247, No. 5,501,359, etc.
The GTT No. 96 type storage tank includes primary and secondary sealing walls comprising 0.5˜0.7 mm thick Invar steel (36% Ni), and primary and secondary thermal insulation walls comprising a plywood box and perlite, which are stacked on an inner surface of the hull.
For the GTT No. 96 type, since the primary and secondary sealing walls have substantially the same liquid-tight properties and strength, it is possible to ensure safety in sustaining a cargo for a significantly long period of time even after the primary sealing wall is damaged to cause leakage of the cargo. Further, since the sealing walls of the GTT No. 96 type are composed of linear membranes, welding can be more conveniently performed than on the Mark III-type composed of corrugated membranes, thereby providing a higher degree of welding automation and a greater overall welding length than the Mark III-type. Further, the GTT No. 96 type employs a double couple to support heat-insulating boxes, that is, the thermal insulation walls.
The Mark III-type storage tank includes a primary sealing wall composed of a 1.2 mm thick stainless steel membrane, a secondary sealing wall composed of a triplex, and primary and secondary thermal insulation walls composed of polyurethane foam and the like, which are stacked on an inner surface of the hull.
For the Mark III-type, the sealing walls have a corrugated part which absorbs contraction by LNG stored in a cryogenic state, so that large stress is not generated in the membrane. For the Mark III-type, a heat-insulating system does not allow structural reinforcement due to the internal structure thereof and the secondary sealing wall does not sufficiently ensure prevention of LNG leakage compared to the secondary sealing wall of the GTT No. 96 type.
Since the membrane type LNG storage tank has lower strength than the independent type storage tank due to the structural characteristics thereof, the membrane type LNG storage tank is very vulnerable to liquid sloshing. Herein, the term “sloshing” refers to movement of a liquid material, that is, LNG, accommodated in the storage tank while a vessel sails in various marine conditions. The wall of the storage tank is subjected to severe impact by sloshing.
Since such a sloshing phenomenon inevitably occurs during voyage of the vessel, it is necessary to design the storage tank to have sufficient strength capable of sustaining the impact force by sloshing.
FIG. 1 shows one example of a conventional liquefied gas storage tank 10 that has upper and lower chambers 11, 12 slanted at about 45 degrees at upper and lower lateral sides of the storage tank 10 to reduce an impact force by sloshing of LNG, particularly, a sloshing impact force in a lateral direction.
For the conventional storage tank 10, the chambers 11, 12 are formed at the upper and lower lateral sides thereof, thereby partially solving problems relating to the sloshing phenomenon. However, as LNG carriers gradually increase in size, the size of the storage tank 10 also increases and the impact force by sloshing becomes severe.
As such, with increasing size of the storage tank, there are demands for solving the problem of an increase in impact force by sloshing and for reinforcing the storage tank to support load of an upper structure of the carrier.
Recently, with gradually increasing demands for floating marine structures such as LNG FPSO (Floating, Production, Storage and Offloading), LNG FSRU (Floating Storage and Regasification Unit) or the like, there is a demand for solving the sloshing problem and the load problem of the upper structure for the liquefied gas storage tanks provided to such floating marine structures.
The LNG FPSO is a floating marine structure that permits direct extraction and liquefaction of natural gas into LNG at sea to store the LNG in the storage tanks thereof and to deliver the LNG stored in the storage tanks to another LNG carrier, as needed. The LNG FSRU is a floating marine structure that permits storage of LNG, discharged from an LNG carrier, in the storage tanks at sea a long distance from land and gasification of the LNG as needed, thereby supplying the regasified LNG to consumers on the land.
Korean Patent No. 0785475 (Hereinafter, Document 1) discloses a storage tank that is provided with a structure (that is, a bulkhead), such as partitions, inside the storage tank to divide an interior space of the storage tank into several spaces, instead of increasing the size of the storage tank, thereby providing the effect of installing several storage tanks each having a small capacity and solving the sloshing problem.
FIGS. 2 and 3 show a storage tank 20 that is disclosed in Document 1 and includes the partition-shaped structure to divide the interior space of the storage tank 20 into two spaces in order to reduce the influence of sloshing.
As shown in FIGS. 2 and 3, the storage tank 20 of Document 1 includes an anti-sloshing bulkhead 23 dividing the interior of the storage tank 20 and stools 25 bonded at one side thereof to an inner wall 21 of a hull and bonded at the other side thereof to the anti-sloshing bulkhead 23 to secure the anti-sloshing bulkhead 23 inside the storage tank.
Each of the stools 25 includes thermal insulation pads 26 connected to primary and secondary barriers 22a, 22b of the storage tank 20, respectively, to prevent leakage of the cryogenic liquefied gas or heat transfer to the inner wall of the hull.
For the storage tank of Document 1, however, since a single storage tank 20 is divided into several spaces by the anti-sloshing bulkhead 23, there is a problem in that the anti-sloshing bulkhead 23 is not firmly secured inside the storage tank to sufficiently absorb the sloshing impact.
Namely, to allow the partition-shaped structure, that is, the anti-sloshing bulkhead 23, to be firmly secured inside the storage tank 20 so as to absorb the sloshing impact, the stool 25 must be firmly disposed between the anti-sloshing bulkhead 23 and the inner wall 21 of the hull. To this end, the stool 25 is made of a sufficiently thick metal plate or includes a number of connection points with respect to the inner wall 21 of the hull.
In this case, however, there is a high possibility that the amount of heat transferred from an exterior into the storage tank 20 increases, thereby deteriorating thermal insulation performance of the storage tank 20 while generating a great amount of boil-off gas within the storage tank 20.
On the other hand, if the thickness of the metal plate for the stool 25 is decreased or the number of connection points between the stool 25 and the inner wall 21 of the hull is decreased to enhance thermal insulation performance of the storage tank 20, the connection points between the anti-sloshing bulkhead 23 and the stool 25 or the connection points between the stool 25 and the inner wall 21 of the hull can be damaged due to the sloshing impact.
Further, the stools 25 provide discontinuous points on the primary and secondary barriers of the storage tank 20, which cause damage of the primary and secondary barriers by thermal shrinkage or expansion of the storage tank 20.
Moreover, since the anti-sloshing bulkhead 23 is the partition-shaped thin structure, it cannot support load from an upper deck of the marine structure.